(a) Field of the Invention
The present invention relates to a method and apparatus for embodying and synchronizing downlink signals in a mobile communication system, and a method for searching cells using the same. More specifically, the present invention relates to a method for generating a structure of a preamble and a pilot appropriate for downlinks of an OFDMA (orthogonal frequency division multiplexing access)-based cellular system, performing synchronization, and searching cells at a terminal by using the structure.
(b) Description of the Related Art
In general, a terminal is required to read signals of a base station and synchronize its time and frequency with the terminal for initial synchronization, and search cells in a cellular system. When initially synchronized, the terminal is also needed to track the time and frequency, synchronize time and frequency of adjacent cells, and search the cells thereof for handover.
Downlinks for enabling initial synchronization, cell search, tracking, and adjacent cell search are provided to the GSM which is a conventional TDMA (time division multiplexing access)-based cellular system, or the IS-95, cdma2000, and W-CDMA which are CDMA (code division multiplexing access)-based cellular systems.
For example, in the W-CDMA system, a P-SCH (primary synchronization channel) and an S-SCH (secondary synchronization channel) of 256-chip lengths are provided for each slot start point so that slot synchronization may be estimated by using the P-SCH, and a scrambling code group number and frame synchronization may be estimated by using the S-SCH. In this instance, a time for estimating the synchronization is minimized by using the P-SCHs of the same pattern for respective slots of each cell, and frame synchronization and a scrambling code group are estimated by using a different pattern per 64 different scrambling code groups and using a different pattern per slot. A P-CPICH (primary common pilot channel) is used to find one of eight major scrambling codes within a scrambling code group, the scrambling code is used to demodulate cell information provided on a P-CCPCH (primary common control channel) and obtain the cell information, and hence, the cell search is finished.
Conventional OFDMA-based systems include the DAB (digital audio broadcasting), the DVB (digital video broadcasting), the IEEE802.11a, and the Hiperlan/2. The DAB uses a null symbol and a phase reference symbol for frame synchronization, and the DVB uses a pilot for the frame synchronization. Also, the IEEE802.11a and the Hiperlan/2 use a preamble to synchronize downlink burst. However, it is difficult for the OFDMA-based cellular systems to perform synchronization and search the cells through the conventional structure since the OFDMA-based systems are not cellular systems.